Re: [V2 PATCH] tmpfs: add fallocate support

[Hugh Dickins <hughd@google.com>]

On Fri, 18 Nov 2011, Cong Wang wrote:

> It seems that systemd needs tmpfs to support fallocate,
> see http://lkml.org/lkml/2011/10/20/275. This patch adds
> fallocate support to tmpfs.
> 
> As we already have shmem_truncate_range(), it is also easy
> to add FALLOC_FL_PUNCH_HOLE support too.

Thank you, this version looks much much nicer.

I wouldn't call it bug-free (don't you need a page_cache_release
after the unlock_page?), and I won't be reviewing it and testing it
for a week or two - there's a lot about the semantics of fallocate
and punch-hole that's not obvious, and I'll have to study the mail
threads discussing them before checking your patch.

First question that springs to mind (to which I shall easily find
an answer): is it actually acceptable for fallocate() to return
-ENOSPC when it has already completed a part of the work?

But so long as the details don't end up complicating this
significantly, since we anyway want to regularize the punch-hole
situation by giving tmpfs the same interface to it as other filesystems,
I now think it would be a bit perverse to disallow the original
fallocate functionality that you implement here in-kernel.

Thanks,
Hugh

> 
> Cc: Pekka Enberg <penberg@kernel.org>
> Cc: Hugh Dickins <hughd@google.com>
> Cc: Dave Hansen <dave@linux.vnet.ibm.com>
> Cc: Lennart Poettering <lennart@poettering.net>
> Cc: Kay Sievers <kay.sievers@vrfy.org>
> Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> Signed-off-by: WANG Cong <amwang@redhat.com>
> 
> ---
>  mm/shmem.c |   43 +++++++++++++++++++++++++++++++++++++++++++
>  1 files changed, 43 insertions(+), 0 deletions(-)
> 
> diff --git a/mm/shmem.c b/mm/shmem.c
> index d672250..96bf619 100644
> --- a/mm/shmem.c
> +++ b/mm/shmem.c
> @@ -30,6 +30,7 @@
>  #include <linux/mm.h>
>  #include <linux/export.h>
>  #include <linux/swap.h>
> +#include <linux/falloc.h>
>  
>  static struct vfsmount *shm_mnt;
>  
> @@ -1431,6 +1432,47 @@ static ssize_t shmem_file_splice_read(struct file *in, loff_t *ppos,
>  	return error;
>  }
>  
> +static long shmem_fallocate(struct file *file, int mode,
> +				loff_t offset, loff_t len)
> +{
> +	struct inode *inode = file->f_path.dentry->d_inode;
> +	pgoff_t start = offset >> PAGE_CACHE_SHIFT;
> +	pgoff_t end = DIV_ROUND_UP((offset + len), PAGE_CACHE_SIZE);
> +	pgoff_t index = start;
> +	loff_t i_size = i_size_read(inode);
> +	struct page *page = NULL;
> +	int ret = 0;
> +
> +	mutex_lock(&inode->i_mutex);
> +	if (mode & FALLOC_FL_PUNCH_HOLE) {
> +		if (!(offset > i_size || (end << PAGE_CACHE_SHIFT) > i_size))
> +			shmem_truncate_range(inode, offset,
> +					     (end << PAGE_CACHE_SHIFT) - 1);
> +		goto unlock;
> +	}
> +
> +	if (!(mode & FALLOC_FL_KEEP_SIZE)) {
> +		ret = inode_newsize_ok(inode, (offset + len));
> +		if (ret)
> +			goto unlock;
> +	}
> +
> +	while (index < end) {
> +		ret = shmem_getpage(inode, index, &page, SGP_WRITE, NULL);
> +		if (ret)
> +			goto unlock;
> +		if (page)
> +			unlock_page(page);
> +		index++;
> +	}
> +	if (!(mode & FALLOC_FL_KEEP_SIZE) && (index << PAGE_CACHE_SHIFT) > i_size)
> +		i_size_write(inode, index << PAGE_CACHE_SHIFT);
> +
> +unlock:
> +	mutex_unlock(&inode->i_mutex);
> +	return ret;
> +}
> +
>  static int shmem_statfs(struct dentry *dentry, struct kstatfs *buf)
>  {
>  	struct shmem_sb_info *sbinfo = SHMEM_SB(dentry->d_sb);
> @@ -2286,6 +2328,7 @@ static const struct file_operations shmem_file_operations = {
>  	.fsync		= noop_fsync,
>  	.splice_read	= shmem_file_splice_read,
>  	.splice_write	= generic_file_splice_write,
> +	.fallocate	= shmem_fallocate,
>  #endif
>  };
>  

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